Question

The electro-positive character decreases from B to Al and then increases from Al to Tl because of:

• A. decrease in ionization energy of the elements
• B. increase in size of the elements
• C. ineffective shielding of nuclear charge by d-electrons in case of Ga, In and Tl
• D. decrease in electronegativity of the elements

Answer 1

The correct option is C ineffective shielding of nuclear charge by d-electrons in case of Ga, In and Tl

The electro-positive character decreases from B to Al and then increases from Al to Tl because of ineffective shielding of nuclear charge by d-electrons in case of Ga, In and Tl.
Due to the small size of boron, the sum of its first three ionization enthalpies is very high. This prevents it to form +3 ions and forces it to form only covalent compounds. But as we move from B to Al, the sum of the first three ionization enthalpies of Al considerably decreases and is, therefore, able to form $A{l}^{3-}$ ions. In fact, aluminium is a highly electropositive metal. However, down the group, due to the poor shielding effect of intervening d and f orbitals, the increased effective nuclear charge holds ns electrons tightly, restricting their participation in bonding. As a result of this, an only p-orbital electron may be involved in bonding. In fact in Ga, In and Tl, both +1 and +3 oxidation states are observed.